Hollow plate heat exchangers are well known in the art and are widely used where large exchange areas in a small volume are desirable. They have been fabricated in both metal and plastic for doing various jobs. In spite of he recognized superiority of welded heat exchangers, nearly all the hollow plate heat exchangers of the prior art are held together by caulk, adhesives, or gaskets usually with mechanical clamping means. In the case of metal exchangers, seal has been accomplished by using lower melting solders or brazing alloys. The reason for this is the great difficulty of making the very long welds associated with flat plate heat exchangers and the difficulty of melting together the light and close spaced structure of the hollow plates without destroying the structure or closing up the openings in the structure rendering it useless for heat exchange. An all-welded monolithic plastic heat exchanger would be desirable since to avoids the use of sealants or adhesives which are usually less resistant to the environment and temperature than the base plastic of which the plates are made. It avoids loss of operability of the structure which could delaminate due to the harsh environment of the exchangers and the differential expansion of the material of the exchanger plates and the sealant or temperature cycling. Finally it avoids the need for mechanical clamping of the structure necessary with many designs, thus avoiding complications and increased weight and susceptibility to degradation of the clamping means. Welding thus gives a better heat exchanger at usually a lower cost than other fabricating techniques.
Many attempts have been made to weld together hollow plastic plate heat exchangers, by use of hot gas or hot plate welding. These have universally failed except for very small exchangers, because of destruction of the delicate structure of hollow plastic plates or closing up of the holes in the closely spaced skins of the sheet by the melted plastic.
Success at making large flat seals necessary for the welding of hollow plate heat exchangers has been achieved by embedding electrical wires in the seal area and then melting controlled amounts of plastic to form a seal even in very fragile structures without excessive distortion. This method is expensive and results in the presence of the melt wire in the seal area which could lead to problems of corrosion or temperature cycling. Another technique is the incorporation of a film of the same plastic as the sheets to which has been added an electromagnetic susceptor between the surface of the hollow sheet and the spacer. By exposure to the proper electromagnetic energy source, the film and the surface of the hollow sheet and the spacer are rendered molten in a controlled fashion without distorting the structure of the hollow sheet. This technique is expensive and would be very difficult to apply to large multi-plate heat exchangers, which will be the most useful.
Hot wire and hot knife techniques are used for sealing and cut-off of film structures to form bags, pillows, and similar structures from film. They have not been applied to welding of massive structures since they give a very shallow seal and are difficult to use in the presence of massive sections of polymer.
It is plainly desirable and technically very difficult to weld hollow plastic plate heat exchangers and a novel method of doing it is the basis for this invention.